1. Field of the Invention
The present invention relates to processes for fabricating mirrors suitable for use in precision instruments such as ring laser gyroscopes. The present invention also relates to processes for depositing multiple coatings on a substrate, such as a mirror substrate. More particularly this invention pertains to a process for controlling the thickness of mirror layers deposited upon a mirror substrate within a deposition chamber.
2. Description of the Prior Art
A ring laser gyroscope is a rotation sensor that senses rotation about an axis that is perpendicular to the plane of a cavity formed within a frame, preferably of glass ceramic or other low thermal coefficient material. Highly polished mirrors are positioned at the corners of the cavities to direct the counterpropagating beams about the cavity. Beams of laser light circulate in opposite directions within the cavity. In accordance with the well-known Sagnac effect, the frequencies of the two beams are altered in opposite senses (i.e. one is increased while the other is decreased) by rotation about the axis and the beat frequency between the two beams then provides a measure of rotation. Lasing is affected within the cavity by the interaction of photons with an excited medium which acts as an amplifier. In a d.c. configuration, the medium is excited by the interaction of a fill gas, typically HeNe, with flows of electrical current between electrodes arranged about or within the gyro frame. Alternatively, in an r.f. actuated device, the medium is excited by means of an electromagnetic field that oscillated at radio frequencies. Only two counterrotating lasing modes need to be supported within the lasing cavity to obtain a measure of rotation. In a planar cavity, the counterrotating beams are linearly polarixed whereas a nonplanar cavity can support both right and left circularly-polarized modes.
As mentioned above, mirrors are provided at the comers of the cavity for directing the beams of light. In the case of a multioscillator or other multiple-cavity device the number of mirrors will correspondingly increase. The precision and, for that matter, operability of a ring laser gyroscope is critically dependent upon the quality of the mirrors. Surface defects and unevenness can produce a multitude of device infirmities.
The fabrication of high quality high reflectance mirrors for use in precision instruments such as ring laser gyroscopes involves the careful deposition of multiple layers of various material composition. Multiple layers are required to provide the high reflectivity that is necessary to generate the feedback required for ring laser gyro operation. Without the necessary high reflectivity, the gyro may be unable to assume operation as a low gain oscillator. That is, successful gyro operation requires that gyro gain exceed losses for the desired oscillation to occur.
The need to deposit multiple layers of material multiplies the possible sources of mirror surface nonuniformity. The presence of nonuniform mirror layers may result in the creation of a color band across the mirror aperture reflecting a spatial distribution of frequency response that does not permit laser gyro operation.
The coating layers of a precision mirror are created through a variety of deposition processes. Such processes are conventionally performed within a coating chamber that essentially comprises a sealable vacuum box. Among the processes for depositing materials in such an environment are thermal evaporation, electron beam, ion beam, and magnetron sputtering. FIG. 1 is a schematic view of the application of a mirror layer by one of such deposition processes within a coating chamber. Within the chamber, mirror substrates 10 are mounted upon a generally-planar rotatable tool 12 for receiving deposited material 14 produced by a material source (not shown). The physics of each of the above-named processes is characterized by the generation of an inherently-nonuniform spatial distribution of coating material known as a "plume".
The inherently non-uniform shape of the plume generated by the material source complicates the task of depositing mirror layers of uniform thickness. Even with rotation of the tool 12, each layer will have different thicknesses at different locations along the radius of the tool 12.
In accordance with the prior art, a shadow mask is placed between the material source and the tool 12. The more complete schematic view of a chamber deposition process illustrated by FIG. 2 shows such a shadow mask 16 in front of the tool 12. The mask is typically formed of thick (1/8-1/4 inch) aluminum or other durable, solid material. The shadow mask 16 is shaped to even out the distribution of material that reaches the tool 12. The shape of the shadow mask 16 is successively trimmed until the desired uniformity of layer deposition in the presence of the plume-like emission of material is observed. Those skilled in the art will recognize that the mask shape is determined empirically. The most appropriate shape will depend on many factors, including the material being deposited, the configuration of the vacuum deposition chamber, and others.
It is well known by those skilled in the art that multiple fixed masks may be used to produce uniformity over surfaces with complex shapes. Several masks may be placed in various positions within the vacuum deposition chamber during a single deposition process to provide the appropriate mask shape to compensate for both a particular deposition material plume shape, and the complex shape of the surface being coated.
The mask 16 is held rigidly in a fixed position within the coating chamber throughout the deposition process.